build: use full paths on compile/link targets

[kmk3]

This makes the compile commands clearer when building in parallel (with
make -j) and ensures that __FILE__ includes the full build-time path
(relative to the root of the repository) whenever it is referenced, such
as in failed assert() messages (currently the full path is only shown in
errExit() messages). Example:

Before:

firejail: main.c:100: main: Assertion `1 == 2' failed.
Error src/firecfg/main.c:100: main: malloc: Cannot allocate memory

After:

firejail: ../../src/firejail/main.c:100: main: Assertion `1 == 2' failed.
Error ../../src/firecfg/main.c:100: main: malloc: Cannot allocate memory

Commands used to search and replace:

$ git grep -Ilz '^MOD_DIR =' -- '*Makefile' | xargs -0 -I '{}' \
  sh -c "printf '%s\n' \"\$(sed -E \
    -e 's|^MOD_DIR = src/(.*)|MOD = \\1\\nMOD_DIR = \$(ROOT)/src/\$(MOD)|' \
    -e 's:^(PROG|SO) = [^.]+(\.so)?$:\\1 = \$(MOD_DIR)/\$(MOD)\2:' \
    '{}')\" >'{}'"
$ git grep -Ilz '^HDRS :=' -- '*.mk' | xargs -0 -I '{}' \
  sh -c "printf '%s\n' \"\$(sed -E \
    -e 's|wildcard (\*\..)|wildcard \$(MOD_DIR)/\\1|' '{}')\" >'{}'"

Note: config.mk.in, src/fnettrace/Makefile and src/include/common.h were
edited manually.

This is a follow-up to #5871.